The present invention relates to a process for producing a high-purity hafnium amide (e.g., tetrakis(diethylamido)hafnium Hf[N(C2H5)2]4) as a hafnium complex, which is preferable as a film-formation raw material in the semiconductor production. Specifically, hafnium amide can be used as a thin-film material for forming a gate insulating film of large-scale integrated circuits (LSI) or the like in metal organic chemical vapor deposition (MOCVD) process.
Hafnium amide as a hafnium complex is relatively high in volatility, and its uses have expanded in recent years as a raw material for forming a hafnium thin film of semiconductor gate insulating films in MOCVD processes. Since the gate insulating film is positioned at the bottom of a semiconductor, it is required to have an extremely high-purity film composition. Thus, the hafnium amide for that is also required to be a high-purity product.
In general, hafnium amide is produced by using hafnium tetrachloride as a raw material. Of impurities contained in hafnium amide, zirconium component, which is derived from this raw material, is generally contained in a high concentration of about 1,000-5,000 weight ppm (wtppm). Since hafnium and zirconium belong to the same group of the periodic table and have analogous chemical properties due to lanthanide contraction, it is difficult to separate hafnium and zirconium from each other. Therefore, zirconium component is generally contained in the above-mentioned high concentration in hafnium amide.
Zirconium component contained in a hafnium amide is a zirconium amide having the same ligands as those of the hafnium amide. Tetrakis(diethylamido)hafnium, for example, produced by using hafnium tetrachloride and lithium diethylamide as raw materials contains tetrakis(diethylamido)zirconium as a zirconium component, which is derived from zirconium tetrachloride contained in the hafnium tetrachloride. Since tetrakis(diethylamido)hafnium and tetrakis(diethylamido)zirconium have almost no difference in vapor pressure and have similar chemical properties, their separation is difficult by a simple procedure, such as simple distillation under reduced pressure (see D. M. Hausmann et al., “Chem. Mater.”, Vol. 14, No. 10, 4350-4358 (2002)).
T. Otsuka, “Journal of the Mining Institute of Japan” (written in Japanese with English abstract at the last page) pages 993-999 (1969) proposes a zirconium and hafnium separation by liquid-liquid extraction with hexone (methyl isobutyl ketone). This separation is high in running costs, since recovery of the solvent and the reagents used is inferior. Furthermore, it is difficult to apply the separation to hafnium amide, which has a hydrolysis tendency, since the separation uses an extraction distribution coefficient difference between water and hexone.
Japanese Patent Laid-open Publication 2005-298467 discloses a process for purifying a hafnium amide, tetrakis(dimethylamido)hafnium, by removing zirconium component through a fractional distillation under reduced pressure. The hafnium amide of this publication is limited to tetrakis(dimethylamido)hafnium due to the relation of vapor pressure difference between hafnium component and zirconium component. In other words, the process of this publication cannot be applied to the purifications of other hafnium amides, such as tetrakis(ethylmethylamido)hafnium and tetrakis(diethylamido)hafnium.
Japanese Patent Laid-open Publication 2005-263771 discloses a process for producing a high-purity tetrakis(dialkylamino)hafnium by the steps of (a) reacting hafnium tetrachloride with a dialkylaminolithium in an organic solvent having a water content of 100 ppm or less, under an inert gas atmosphere; and (b) subjecting a crude tetrakis(dialkylamino)hafnium obtained by the step (a) to fractional distillation.